The following U.S. patents are believed to represent the current state of the art: U.S. Pat. Nos. 5,159,321; 5,247,137; 5,294,792; 5,434,371; 5,453,758; 5,517,579; 5,587,558; 5,598,187; 5,781,661; 5,902,968; 5,981,884; 6,084,557; 6,130,666;
The following PCT International Publications are believed to represent the current state of the art: WO 94/09447; WO 94/18663; WO 95/21436; WO 99/22338;